Contestation over the structure and location of final sovereign authority—the right to make and enforce binding rules—occupies a central role in political development. Historically, war often settled these debates and institutionalized the victor’s vision of sovereignty. Yet sovereign authority requires more than institutions; it ultimately rests on the recognition of the governed. How does war shape imagined sovereignty? We explore the effect of warfare in the United States, where the debate over two competing visions of sovereignty erupted into the American Civil War. We exploit the grammatical shift in the “United States” from a plural to a singular noun as a measure of imagined sovereignty, drawing upon two large textual corpuses: newspapers (1800–99) and congressional speeches (1851–99). We demonstrate that war shapes imagined sovereignty, but for the North only. Our results further suggest that Northern Republicans played an important role as ideational entrepreneurs in bringing about this shift.